1. Field of the Invention
The invention concerns a method and device for acquisition of seismic data.
2. Description of the Prior Art
Seismic data is currently acquired in a prospecting area by means of an array of receivers disposed on or over the terrestrial surface in the prospecting area. A series of firings is executed to generate excitation vibrations, hereinafter referred to simply as vibrations, the vibrations generated propagating towards the underlying terrestrial strata and being reflected therefrom. A single source having a vibration emission frequency spectrum covering the usable signal range, which is a frequency range or band between 10 Hz and 60 Hz, is usually employed.
For a single-row array R as shown in FIG. 1a in which the geophones or groups of geophones G1, G2 are spaced by a distance e, the single frequency band source S is placed at the center of the array R and the maximum distance to the end geophone (the "remote-on" distance) is designated L.
The time-distance diagram of vibration echoes reflected by underlying terrestrial strata as shown in FIG. 1a representative of the group propagation speed of the reflected vibrations features, for the geophones or groups of geophones disposed in the vicinity of the source S and because of saturation at the low-frequency components of the latter, a disturbance due to the slow waves corresponding to the vibrations with low propagation speed, which are therefore steeply inclined in the FIG. 1a diagram. The effect of this disturbance is to mask and jumble with low-frequency noise the response to the high-frequency vibration components of the geophones or groups of geophones situated in the vicinity of the source S, these high-frequency components being much less steeply inclined in the FIG. 1a diagram.
An advantageous representation as in FIG. 1b of the relative levels of the low-frequency noise signals and the usable signal S in a frequency-time domain of the spectrum of the emitted vibrations, spatial frequency or spatial wave number K and spatial resolution of the array shows that the low-frequency noise level denoted GR corresponds to a maximum relative level of the signal between 0 dB and -5 dB in the corresponding F, K(GR) domains whereas the corresponding level in the corresponding F, K(S) domain, which is substantially trapezium-shaped, corresponds to a corresponding relative level between -10 dB and -30 dB. Including the noise signal in a processing domain corresponding to the rectangle of FIG. 1b diagram (1) therefore raises the problem previously explained with reference to FIG. 1a.
A first solution to this problem might consist in temporal frequency filtering of the signals received by the geophones in response to the vibrations, as represented by the smaller rectangular domain of FIG. 1b diagram (2). Although this solution retains the maximal spatial resolution for the high-frequency signals, it has the disadvantage of limited temporal frequency response with corresponding loss of information.
Another solution to this problem might consist, as shown in FIG 1b diagram (3), in retaining all temporal frequency components of the signals received by the geophones in response to the vibrations. However, this could only be achieved at the cost of reducing the distance resolution of the array corresponding to the lower spatial wave number limit of the noise domain GR.
As shown in FIG. 1b diagram (4), an intermediate solution might consist in limiting both the usable signal component frequency band and the spatial resolution of the array. However, this solution also leads to global loss of information from the echo signals.
An object of the invention is to remedy the aforementioned disadvantages by employing a method whereby it is possible both to retain the vibration component frequency band and to increase the spatial resolution of the array of receivers.
Another object of the present invention is to provide a method offering improved spatial resolution performance from the same quantity of equipment as in the prior art methods.
A further object of the invention, given the use of the same transmission-reception equipment as in the prior art methods, is the use of operating conditions that are similar from the points of view of the vibration force applied and the quantity of information acquired per trace.